Chart 1. Select Ligands and Radioligands Used to Study the A₃ARa.

^a Agonists 3 and 5 are used in radioiodinated form, and antagonists 7 and 8 have been tritiated. Binding K_i values at the hA₃AR (nM) are: 1, 1.8; 2, 1.4; 3, ∼1; 4, 0.70; 5, 1.4; 6, 43.9; 7, 1.13; 8, 3.51.^1,13